Acceleron, anyone?

July 27, 2004If there is a phantom in physics, then the neutrino is it. The nuclear fires of the stars form these subatomic particles by the trillions every second, yet neutrinos stream unperturbed through the cosmos at nearly the speed of light, their passage through planets and people almost entirely unnoticed.

During the past decade, particle physicists have devised experiments that reveal the neutrino is a bit more substantial than previously thought. Over the same period, astronomers have found that the expansion of the universe is accelerating and that the bulk of energy in the cosmos is "dark energy" of an undetermined nature. Could all these things be linked?

A trio of University of Washington physicists thinks they are. The missing piece of the puzzle, says physicist Ann Nelson and her team, is an as-yet-undiscovered subatomic particle they call the "acceleron."

The main ingredient of the universe is "dark energy," a mysterious form of energy that exists between galaxies and forces the universe to expand at an ever-increasing rate.

Ann Field (STScI)

Similar to an optical lens bending light to form an image, the galaxy cluster Abell 2218 is so massive and compact that light rays passing through it are deflected by its enormous gravitational field. This process (called gravitational lensing) magnifies, brightens, and distorts images of objects that lie far beyond the cluster. The thin arcs are distorted images of distant galaxies roughly 5-10 times farther than the cluster.

A. Fruchter / STScI / NASA

Accelerons are even more reluctant to interact with matter than neutrinos, which is why they haven't been found. But Nelson suggests neutrino experiments already operating around the world could detect their subtle influence.

In the team's scenario, accelerons interact with and influence neutrinos through a new force. In an expanding universe, this new force results in a tension that fuels faster expansion. The physicists argue that both accelerons and neutrinos are components of dark energy. Dark energy now accounts for more than 65 percent of the energy of the universe, but in the early days of the cosmos, it barely registered.

"There are many models of dark energy, but the tests are mostly limited to cosmology, in particular measuring the rate of expansion of the universe. Because this involves observing very distant objects, it is very difficult to make such a measurement precisely," Nelson says. "This is the only model that gives us some meaningful way to do experiments on Earth to find the force that gives rise to dark energy."

Another peculiarity of the neutrino is that its mass can change according to the environment through which it is passing. In Nelson's view, eventually neutrinos would get too far apart and become too massive to fuel cosmic acceleration. "The universe could continue to expand," she says, "but at an ever-decreasing rate."

Nelson and coworkers David Kaplan and Neal Weiner detail their arguments in a paper to appear in a forthcoming issue of Physical Review Letters.